MEDIUM: config: now reject configs with empty arguments

MEDIUM: config: now reject configs with empty arguments

As prepared during 3.2, we must error on empty arguments because they
mark the end of the line and cause subsequent arguments to be silently
ignored. It was too late in 3.2 to turn that into an error so it's a
warning, but for 3.3 it needed to be an alert.

This patch does that. It doesn't instantly break, instead it counts
one fatal error per violating line. This allows to emit several errors
at once, which can often be caused by the same variable being missed,
or a group of variables sharing a same misspelled prefix for example.
Tests show that it helps locate them better. It also explains what to
look for in the config manual for help with variables expansion.